This invention relates to techniques for repairing turbine engines and particularly to a method for circumferentially distributing a population of shroud segments of nonuniform radial thickness in an engine module having a circumferentially nonuniform requirement for segment thickness.
A typical aircraft gas turbine engine includes one or more turbine modules for extracting energy from a stream of working medium fluid. A representative turbine module includes a rotor comprising a rotatable hub disposed about a rotational axis or module centerline. An array of blades projects radially from the hub so that the individual blades span a working medium flowpath. The rotor has a diameter, which is the linear distance between the tips of radially opposing blades. An array of nonrotatable vanes, axially spaced from the blade array, also spans the flowpath. A case circumscribes the blade and vane arrays. The case includes a plurality of circumferentially distributed shroud or airseal segments secured to the interior of the case and axially aligned with the blade tips. Collectively, the segments define a substantially cylindrical shroud or airseal assembly that radially bounds the flowpath in the vicinity of the blade tips. The shroud assembly is radially spaced from the blade tips by a clearance gap. Engine designers strive to minimize the clearance gap because any working medium fluid that leaks through the gap instead of flowing over the surfaces of the blades degrades engine efficiency.
The shroud segments in modern generation engines are made of an abradable material. When a new engine (or one having new or reconditioned shroud segments) is first operated, centrifugal and thermal growth of the blades relative to the case causes the blade tips to carve a trench in the abradable material. Thereafter, the blade tips extend into the trench during engine operation to minimize fluid leakage. By contrast, older generation engines employ nonabradable shroud segments comprising a metallic substrate and a metallic surface layer deposited on the substrate by plasma spraying or other suitable means. The shroud segments and blades are carefully dimensioned to minimize the size of the clearance gap without actually coming into contact with each other.
One accepted method for building a turbine module for an older generation engine is to install a set of radially oversized shroud segments in the case and to then grind the installed segments to a smaller radial dimension compatible with the diameter of the rotor. The grinding operation is carried out with a grinding apparatus whose grinding tool follows a circular grind path at a prescribed radius from a grind centerline. For some turbines, the grind centerline may coincide with the module centerline so that the shroud assembly, as ground, is uniformly radially spaced from the module centerline. In other turbines, the grind centerline may be radially offset from the module centerline so that the radial spacing from the module centerline to the shroud assembly varies from point to point around the circumference of the module. This offset grind technique is used to compensate for predictable distortion of the case, and hence of the shroud assembly, occasioned when the engine is mounted on an aircraft.
The above described offset grind technique presents no problem as long as the installed shroud segments are radially thick enough to fall within the grind radius. For modules containing newly manufactured shroud segments, this condition is easily satisfied by establishing an appropriate lower limit on the radial dimension of the segments. However, when it is necessary to replace shroud segments that have deteriorated during normal use, many engine owners prefer to install refurbished segments rather than more costly, newly manufactured segments. A refurbished segment is one whose deteriorated metallic surface layer has been completely removed and replaced by a replacement surface layer deposited on the substrate metal. One material known to be suitable as a replacement is a nickel base alloy known as MARM-509. The MARM-509 alloy can be deposited on the substrate to a thickness up to about 95 mils. A segment refurbished with MARM-509 can therefore be made thick enough to fall within the grind radius of the grinding apparatus irrespective of the angular position where the segment is installed in the turbine case. Another suitable material is a nickel base alloy known as LCO-22. Refurbishment with LCO-22 is considerably less expensive than refurbishment with MARM-509, but the LCO-22 material can be successfully deposited on the metallic substrate up to a thickness of only about 50 mils. As a result, a segment refurbished with LCO-22 may not be universally suitable for use at all positions around the circumference of the case since, at some positions, it may be too thin to fall within the grind radius. This is undesirable because an unground segment introduces a discontinuity into the otherwise smoothly curved profile the shroud assembly. Moreover, the individual members in a group of refurbished segments are not all of equal thickness, thus complicating any attempt to strategically position the segments in the case. Nevertheless, many engine owners prefer the more economical LCO-22 refurbishment over the more expensive MARM-509 refurbishment.
Alternatively, an engine owner may elect to replace deteriorated shroud segments with serviceable, previously used segments. Previously used segments may be radially thinner than new segments due to erosion of the metallic surface layer during service or because of refinishing operations in which some of the surface layer has been removed to eliminate minor surface flaws such as cracks and pits. In addition, the individual members in a group of used segments differ in thickness. Thus, used segments present the same problems as LCO-22 refurbished segments.
According to past practice, used or refurbished segments are installed randomly in a turbine case. Subsequent to the above described offset grinding operation, a technician inspects the case to determine whether or not the grinding apparatus has ground all the segments. If not, as is often true if used or LCO-22 refurbished segments are employed, the segments are rearranged and/or substitute segments are installed in place of existing segments. The grind operation and subsequent inspection are repeated, but still with no guarantee of success. In extreme situations, several repetitions of the procedure might be required to achieve success. This repetitive process is tedious, time consuming and thus thoroughly unsatisfactory. The likelihood of success can be improved by using new segments or segments refurbished with MARM-509 as the substitute segments. However, either of these options increases the overall cost and threatens the economies sought by employing used or LCO-22 refurbished segments.
What is needed is a systematic method for allocating shroud segments among a set of positions in a turbine case to ensure that the largest possible quantity of segments falls within a prescribed grind radius and for identifying any unsuitable segments.
It is, therefore, an object of the invention to provide a method for systematically allocating a population of shroud segments among a set of positions in a turbomachinery case so that the largest possible number of segments fall within a prescribed grind radius.
According to the invention, a representative radial thickness associated with each segment selected from a population of segments is assigned to an angular position on the case, and each thickness is compared to the minimum thickness requirement of its assigned position. Any thickness value that fails to satisfy the minimum thickness requirement of its assigned position is consigned to a pool of available thickness values, thus vacating zero or more angular positions. The members of the pool are then evaluated for suitability in the vacated positions and members are assigned to vacated positions for which they are suitable.
The principal advantage of the invention is that it quickly identifies an optimum distribution of segments among the various angular positions in the module and therefore reduces the expense and time required to service a turbomachinery module. Accordingly, the method overcomes the disadvantages of employing used or inexpensively refurbished segments and so makes it more practical to use such segments.
The advantages and the operation of the inventive method will become clearer in view of the following description of the best mode for carrying out the invention and the accompanying illustrations.